Brazing filler metal composition and process

ABSTRACT

Disclosed is a brazing filler metal composition comprising, by weight, about 8% to 11% chromium, 2.0%-3.0% boron, 3.0%-4.5% silicon, 2.5%-4.0% iron, 7.0%-9.0% tungsten, a maximum of about 0.06% carbon and the remainder nickel. Further disclosed is a vacuum brazing process utilizing said filler metal composition.

This is a division of application Ser. No. 281,793 filed July 9, 1981,now U.S. Pat. No. 4,394,347.

BACKGROUND OF THE DISCLOSURE

I. Field of the Invention

This invention relates, in general, to brazing filler metals and, morespecifically, to filler metal compositions used to fill holes and repairdamage in turbine engine high temperature components. In particular, theinvention relates to novel filler metals which provide excellent hightemperature corrosion and abrasion resistance. The invention furtherrelates to a vacuum brazing process utilizing said filler metals.

II. Description of the Prior Art

Brazing consists of joining base metal surfaces by fusing a fillermetal, having a lower melting point than the subject base metal, withoutappreciable fusion of the base metal surfaces themselves. For brazing, aflux may be applied to the subject base metal surfaces either prior toor simultaneously with the filler metal.

A satisfactory brazing flux flows at a temperature somewhat below themelting point of the filler metal; adheres to or wets the base metalsurfaces; facilitates the flow and wetting of the filler metal over thesubject base metal surfaces generally by reducing the surface tension ofthe molten filler metal; removes any oxide coating or other adherentforeign matter present on the subject base metal surfaces withoutappreciably attacking the base metal surfaces; inhibits re-oxidation ofthe subject base metal surfaces; and is capable of ready displacement byliquid filler metal either leaving no residue or leaving a readilyremovable, relatively inert residue after completion of the brazing.

Furnace brazing in a vacuum with the use of no flux offers severaladvantages. For example, the possibility of flux inclusions areeliminated and, accordingly, blind cavities, tortuous paths, and smallpassageways can be designed into the assembly without regard to fluxremoval or entrapment after brazing. In addition, fluxless vacuumbrazing eliminates the cost of flux and its application, the need forcleaning the assembly after brazing, and potential corrosion ofequipment and pollution of air and water by flux residues or fluxreaction products.

Nickel-base, copper-base, gold-base, palladium-base, and a fewsilver-base filler metals are commonly used in vacuum furnace brazing.Apart from compatability with the base metal, filler metals areinvariably selected for corrosion resistance in specific media andsuitability for service at known operating temperatures.

Known brazing filler metal compositions, however, do not have thedesired properties necessary for use in filling small holes and otherdefects in high temperature superalloys such as those used in turbineengine high temperature components. As a result, engines with smallholes therein lose efficiency and parts must be scrapped. Moreover,known filler metals do not simultaneously give good wetting, but verylimited flow and the ability to bridge defects, at a brazing temperatureof about 1,950° F., so that defects are sealed without filler materialflowing into internal passages in the components. In addition, knownfiller metals do not have the proper wetting and flow characteristics ata brazing temperature of 1,950° F. while also possessing the ability tofill and bridge holes as well as to provide excellent high temperatureand corrosion resistance and, when properly coated, survive in the harshenvironment of a turbine engine.

It is, therefore, an object of this invention to provide a brazingfiller metal composition which is devoid of the above-noteddisadvantages.

It is another object of this invention to provide a brazing filler metalcomposition which has desired properties for use in filling small holesand other defects in high temperature superalloys, such as those used inturbine engine high temperature components.

It is still another object of this invention to provide brazing fillermetals which wet well and yet have very limited flow at a brazingtemperature of about 1,950° F. while possessing the ability to fill andbridge holes.

It is yet another object of this invention to provide brazing fillermetals which provide very good high temperature corrosion and abrasionresistance.

It is still another further object of this invention to provide brazingfiller metal compositions wherein the brazing process may beaccomplished in a single pass.

It is yet another object of this invention to provide a brazing fillermetal which may be overcoated with coating schemes used for hightemperature nickel superalloys.

It is yet another further object of this invention to provide a vacuumbrazing process utilizing novel nickel-base filler metal compositions.

SUMMARY OF THE INVENTION

The foregoing objects and others are accomplished in accordance withthis invention, generally speaking, by (a) providing a brazing fillermetal composition comprising, by weight, from about 8.0% to about 11.0%chromium; from about 2.0% to about 3.0% boron; from about 3.0% to about4.5% silicon; from about 2.5% to about 4.0% iron; from about 7.0% toabout 9.0% tungsten; a maximum of about 0.06% carbon; and the remaindernickel and (b) utilizing said filler metal with a nickel-base superalloyin a vacuum brazing process.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, any suitable metal may bevacuum-brazed using the novel filler metals. described herein. Suitablemetals include for example, superalloys such as nickel-base superalloysused in turbine engine components, among others. While any suitablemetal may be vacuum-brazed using the filler metal of this invention,particularly good results are obtained with nickel-base superalloys.

While any suitable ratio of materials which comprise the filler metalcomposition of this invention may be used, excellent wetting, excellentability to fill holes, and high temperature corrosion and abrasionresistance are achieved by providing a composition comprising, byweight, from about 8.0% to about 11.0% chromium; from about 2.0% toabout 3.0% boron; from about 3.0% to about 4.5% silicon; from about 2.5%to about 4.0% iron; from about 7.0% to about 9.0% tungsten; a maximum ofabout 0.06% carbon and the remainder nickel. A preferred filler metalcomposition comprises, by weight, about 10.0% chromium; about 2.5%boron; about 4.0% silicon; about 3.5% iron; about 8.0% tungsten; about0.04% carbon and the remainder nickel.

Both hot wall retort and cold wall radiant shield furnaces may be usedin vacuum brazing. However, because of inherent advantages, cold wallfurnaces are by far the more widely used. Cold wall furnaces heat fasterand with greater efficiency, and are suitable for use at highertemperatures and vacuum pressures. At higher operating temperatures, theability of the retort of the hot wall furnace to resist collapse isincreasingly dependent on the supporting vacuum surrounding the retort.

The vacuum pumping system should be capable of evacuating a conditionedchamber to a moderate vacuum, such as, for example, about 10⁻³ torr, inabout 1 hour. The temperature distribution within the work being brazedshould be reasonably uniform (i.e., within about ±10° F.).

The filler metals of the present invention may be coated with coatingschemes used for high temperature superalloys. When properly coated,these filler metals survive in the harsh environment of a turbineengine. Depending upon the nature of the base metals to be repaired, avery thin layer of nickel may be plated onto the area needing repairprior to applying the filler metal.

A preferred brazing process for use with the filler metal composition ofthis invention is to (a) heat in a vacuum to about 1,740° F. and holdfor about 1 hour to stabilize load temperature; (b) continue heating toabout 1,950° F. (the brazing temperature) and hold for about 5 minutesso that the filler metal melts, wets and seals the defect; and (c) coolto about 1,800° F. and hold for about 3 hours.

The present invention will be further illustrated by the followingexamples in which parts and percentages are by weight unless otherwisespecified.

EXAMPLE I

A component having a small hole and made of nickel-base superalloy iscleaned by vapor degreasing with common solvent (e.g. 1,1,1trichloroethane). A thin layer of nickel (varying between about0.0004-0.0008 inch in thickness) is plated onto the areas requiringrepair. The component is then rinsed thoroughly and bake-dried. Thefiller metal composition which comprises, by weight, about 10.0%chromium, about 2.5% boron, about 4.0% silicon, about 3.5% iron, about8.0% tungsten, about 0.04% carbon and the remainder nickel is thenapplied. (The filler metal is mixed in a slurry with a nitrocellulosebinder and is applied to the areas to be repaired). The engine componentis then heated to about 1,740° F. and held for one hour. Heating is thencontinued to about 1,950° F. (the brazing temperature) and held forabout five minutes. The temperature is then reduced to about 1,800° F.and held for about 3 hours. After backfilling the chamber withchemically inert gas, the component is removed at about 400° F. and isair cooled. The dry brazed component is ready for use or furtherprocessing as soon as it is cool. Excellent brazing results areachieved.

EXAMPLE II

Example I is repeated with a filler metal composition comprising, byweight, about 8.0% chromium, about 2.0% boron, about 3.0% silicon, about2.5% iron, about 7.0% tungsten, about 0.01% carbon and the remaindernickel. Excellent brazing results are achieved.

EXAMPLE III

Example I is repeated with a filler metal composition comprising, byweight, about 11.0% chromium, about 3.0% boron, about 4.5% silicon,about 4.0% iron, about 9.0% tungsten, about 0.06% carbon and theremainder nickel. Very good results are achieved.

While specific components of the present system are defined in theworking examples above, any of the other typical materials indicatedabove may be substituted in the working examples, if appropriate. Inaddition, many other variables may be introduced into the brazingprocess, such as further purification steps, etc. which may in any wayaffect enhance, or otherwise improve the present process.

While various specifics are given in the present application, manymodifications and ramifications will occur to those skilled in the artupon a reading of the present disclosure. They are intended to becovered herein.

What is claimed is:
 1. A vacuum brazing process comprising (a) applyinga brazing filler metal composition to a hole in a subject base metal,said filler metal composition comprising, by weight, from about 8.0% toabout 11.0% chromium, from about 2.0% to about 3.0% boron, from about3.0% to about 4.5% silicon, from about 2.5% to about 4.0% iron, fromabout 7.0% to about 9.0% tungsten, a maximum of about 0.06% carbon, andthe remainder nickel, and said subject base metal having a highermelting point than said filler metal composition; (b) heating to thebrazing temperature of the filler metal composition; and (c) cooling. 2.The brazing process of claim 1 wherein said filler metal compositioncomprises, by weight, about 10% chromium, about 2.5% boron; about 4.0%silicon; about 3.5% iron; about 8.0% tungsten; about 0.04% carbon andthe remainder nickel.
 3. The brazing process of claim 1 wherein saidsubject base metal is a nickel-base superalloy.
 4. The brazing processof claim 1 wherein the brazing temperature of said filler metalcomposition is from about 1,940° F. to about 1,960° F.
 5. The brazingprocess of claim 1 wherein the brazing temperature of said filler metalcomposition is about 1,950° F.
 6. The brazing process of claim 3 whereinthe brazing filler metal is overcoated with coatings used for nickelsuperalloys.
 7. The brazing process of claim 3 wherein a thin layer ofnickel is plated into said hole prior to application of said brazingfiller metal composition.
 8. The brazing process of claim 3 wherein thebrazing assembly is (a) heated to about 1,740° F. and held for about 1hour; (b) heating is then continued to the brazing temperature and heldfor about 5 minutes; and (c) the temperature is then reduced to about1,800° F. and held to about 3 hours.
 9. The brazing process of claim 8wherein said brazing temperature is about 1,950° F.